Page 93 - Introduction to Marine Engineering

80 Boilers

Furnace wall construction
The problems associated with furnace refractory materials, particularly
on vertical walls, have resulted in two water-wall arrangements without
exposed refractory. These are known as 'tangent tube' and 'monowall' or
'membrane wall'.
In the tangent tube arrangement closely pitched tubes are backed by
refractory, insulation and the boiler casing (Figure 4.6(a)), In the
monowall or membrane wall arrangement the tubes have a steel strip
welded between them to form a completely gas-tight enclosure (Figure
4.6(b)). Only a layer of insulation and cleading is required on the outside
of this construction.

Close pitched tubes

Outer casing
(a) Tangent tube arrangement

Insulation
Cleading
(b) Monowall arrangement
Figure 4.6 Furnace wall construction

The monowall construction eliminates the problems of refractory and
expanded joints. However, in the event of tube failure, a welded repair
must be carried out. Alternatively the tube can be plugged at either end,
but refractory material must be placed over the failed tube to protect the
insulation behind it. With tangent tube construction a failed tube can be
plugged and the boiler operated normally without further attention.